New directions in photodetectors: from new solid-state photomultipliers to effective mass inters

Abstract

Recent advances in superlattice photodetectors are reviewed. In suitably designed superlattice structures hot carriers in the barrier layers can collide with carriers confined or dynamically stored in the wells and Impact ionize them out across the band-edge discontinuity [Fig. 1(a)].1,2This phenomenon can be the basis for a new class of very low-noise avalanche photodiode and solid-state photomultiplier. Photomultiplication measurements in AlInAs/GaInAs, AlSb/GaSb, and InP/GaInAs superlattices provide strong evidence of this effect. In these structures carrier storage in the wells arises from thermal generation processes [Fig. 1(b)]. A large ionization rate ratio has been observed1 (β/α = 20). By appropriately grading the interface of the wells, the storage of electrons can be eliminated, while holes are still confined. This should maximize the ionization rate ratio by minimizing electron initiated multiplication. This structure was grown by MBE in the AlInAs/ GaInAs system and has led to the observation of near-single-carrier-type multiplication (Fig. 2).